1. Technical Field of the Invention
The present invention relates generally to the field of radio communications, and more particularly, to an apparatus and method that compensates for fading in analog amplitude modulated (AM) radio signals.
2. Description of Related Art
A major problem encountered in transmitting radio signals to mobile receivers (or from mobile transmitters) is known as "fading." Fading in a transmitted radio signal can occur in two ways: (1) the transmitted signal is absorbed or blocked by obstacles (e.g., buildings, metal towers, etc.) located between the radio transmitter and receiver, or (2) the transmitted signal is reflected and scattered off numerous obstacles located along the transmission path. The absorbing or blocking type of fading is known as "shadowing" or "log normal" fading. The reflecting or scattering type is known as "multipath" or "Rayleigh" fading.
In Rayleigh fading, the reflected signals take different paths to the receiver and thus arrive out of phase. This means that the received signal is the sum of the reflected signals, which differ in phase (and to some extent in amplitude). These different-phase signals are added as vectors, so at certain times, the vector sum of the signals can be near zero. Consequently, the strength of the received signal at that time is also near zero.
Analog signals are transmitted over radio channels using AM or frequency modulation (FM). For example, analog voice signals are transmitted using AM in mobile telephone systems, such as the Airphone (air-to-ground mobile telephone) System produced by GTE. Commercial and amateur fixed radio broadcast systems also transmit analog voice signals using AM. These AM analog voice transmissions can include conventional AM, single sideband (SSB), double sideband (DSB), or vestigial sideband (VSB) transmissions.
FM is typically used for many cellular mobile transmissions. An advantage of using FM over AM for cellular transmissions is that FM is far less sensitive to fading than AM, because the voice information being carried is encoded as phase changes in the FM transmission, but the modulation envelope remains constant. Since no information is present in the amplitude of the signal, the information can be reconstructed even if the fading causes a loss in amplitude information.
On the other hand, an advantage of using AM over FM is that AM transmissions occupy a smaller portion of the frequency spectrum than FM transmissions. However, SSB and VSB AM transmissions are highly sensitive to fading, because the voice information is being carried in both the phase and amplitude of the transmitted AM signal.
Different methods have been used to render SSB and VSB AM signals less sensitive to Rayleigh fading. One such method inserts a "pilot tone" having a predetermined magnitude into the transmitted AM carrier signal. At the AM receiver, the magnitude and phase of the received pilot tone is detected and used to estimate and compensate for the effects of the fading channel. However, pilot tones cause relatively high carrier signal fluctuations and expand signal bandwidth requirements.
Pilot symbols (as opposed to tones) inserted in digital signal transmissions are being used to compensate for Rayleigh fading. One technique used to insert pilot symbols in digital signal transmissions to compensate for Rayleigh fading has been disclosed in U.S. Pat. No. 5,414,734 to Marchetto et al ("Marchetto"). Marchetto discloses the use of pilot symbols, which are appended to a plurality of data symbols to form successive frames that are modulated at a transmitter. The received frames are processed to produce a data signal and pilot signal. The data signal is composed of the data symbols, and the pilot signal is composed of the pilot symbols. Channel impulse response estimates are made of successive blocks of the pilot symbols. These estimates are interpolated and applied to successive data symbols in the (delayed) data signal, which enables the receiver to decode the data and compensate for the effects of fading. However, Marchetto's use of pilot symbols for fading compensation is disclosed for digital signals only and not for analog signals.